Turlough was a complex individual. His motivations were often unclear to those around him. He was for a time willingly under the influence of the Black Guardian. During this period, he tried to kill the Doctor. After overcoming the Black Guardian, he was a mostly-trusted member of the TARDIS crew, but there were still moments where his actions were questioned. It was only at the end of his time on the TARDIS that the Doctor finally discovered the truth about how Turlough ended up on Earth and why he was susceptible to the Black Guardian in the first place.

The Doctor once indicated that what made Turlough such an interesting travelling companion was that he never knew what Turlough was thinking.

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Turlough rarely talked about his life on Trion before and during the civil war, although his desperation to destroy a distress beacon he recognised as being from Trion and his reluctance to contact his people to save the inhabitants of Sarn indicated there were severe penalties for defiance of his exile. Before the war, Turlough's family was wealthy, and owned a planet. In the military, he held the rank and serial number of "Junior Ensign Commander Vislor Turlough, VTEC9/12/44". In his teenage years, before he became a member of the military, he fell in love with a girl named Deela. Deela's father was on the opposite side during the civil war. His mother was killed during the civil war and his younger brother Malkon and his father were exiled to the abandoned colony planet of Sarn. Turlough himself was exiled to Earth. (TV: Planet of Fire)

In subsequent adventures, Turlough received several messages from the Black Guardian, reminding him of his obligation to kill the Doctor — an act he became loath to carry out as he grew fonder of the Time Lord. Finally, he destroyed a prize that would have guaranteed his freedom and his pact with the Black Guardian rather than kill the Doctor, ending his association with the Black Guardian and becoming a loyal member of the Doctor's team. (TV: Enlightenment)

Two days later, Turlough, Tegan and the Doctor arrived on Helheim, where they were reunited with Nyssa (although it had been fifty years for her). (AUDIO: Cobwebs)

The Doctor, Turlough, Tegan and Nyssa then arrived on Cherdor in the 28th century, where they found a society which was obsessed with cleanliness, and lived under the menace of the Takers, for which Turlough was mistaken. After discovering the truth behind the origins of the corrupt society, the TARDIS crew returned to the TARDIS, but discovered that Tegan was still possessed by the Mara. (AUDIO: The Whispering Forest)

While travelling with the Doctor, Nyssa, and Tegan, Turlough reunited with his old love Deela. They had a special place that they'd spent time together and Turlough recalled that they'd last been there together "three years, nine months, and seventeen days" ago. (AUDIO: The Kiss of Death)

It was after breaking away from the Black Guardian that Turlough's outlook changed from self-preservation to wanting to help others. This led him to actions such as deliberately putting his own life at risk to save the Doctor and Tegan, (TV: Warriors of the Deep) facing the demons of his own people (TV: Frontios) and finally accepting personal responsibility and trading his freedom for the lives of the population of Sarn. (TV: Planet of Fire)

Although he initially expressed a desire to return to his homeworld and at one point asked the Doctor to take him home, (TV: Enlightenment) after he had spent more time travelling with the Doctor he grew reluctant to return. He destroyed a distress beacon from a Trion ship, attempted to disable Kamelion who he believed was reacting to the beacon and was reluctant to contact his people. (TV: Planet of Fire)

After discovering that his younger brother was still alive and the persecution of political prisoners had long been abandoned on Trion, Turlough finally left the TARDIS to return home, after effectively putting the Doctor ("who gets into the most terrible trouble") in the hands of new companion Peri Brown. (TV: Planet of Fire) Later, as he regenerated, the Doctor had visions of several of his past companions, including Turlough. (TV: The Caves of Androzani)

Tegan did not like Turlough at first, finding him untrustworthy. She did not approve of the Doctor letting Turlough consider the Black Guardian's offer, believing Turlough would choose the crystal. However, the Doctor was confident in Turlough's morals. Nevertheless, Tegan often tried to persuade him to banish Turlough from the TARDIS. (TV: Terminus, Enlightenment; AUDIO: Cobwebs)

Despite the Black Guardian's attempts to vilify the Doctor, he recognised the Doctor's good intentions. On numerous occasions, he tried to make the Black Guardian reconsider his order to kill the Time Lord. (TV: Terminus, Enlightenment) Turlough used the tranquility felt at the Eye of Orion to draw. (TV: The Five Doctors) He understood the TARDIS systems well enough to run a diagnostic (TV: The Five Doctors) and programmed the TARDIS to rescue the Doctor and Peri. (TV: Planet of Fire)

The Doctor once compared Turlough and Tegan to bickering siblings. (AUDIO: Freakshow)